Musnad Ahmad 31:315

I read to Abd al-Rahman: Malik, from Abd al-Rahman ibn Abd Allah ibn Abd al-Rahman ibn Abi Sa'sa'ah al-Mazini, from his father, that he informed him: That Abu Sa'id said to him: "I see that you love sheep and the desert. So when you are among your sheep or in your desert, and you call the Adhan for prayer, raise your voice with the call. For no jinn, nor human, nor anything else hears the extent of the voice of the Mu'adhdhin, except that it will bear witness for him on the Day of Resurrection." I heard it from the Messenger of Allah, may Allah bless him and grant him peace.